When solid dosage forms are taken orally, in many cases, the drug must dissolve in aqueous gastrointestinal fluids in, e.g., the patient's stomach before the drug can exert a therapeutic effect. A recurring problem with compressed solid oral dosage forms, such as tablets and caplets (i.e., capsule-shaped tablets) is that the rate of dissolution of some drugs from the dosage form limits their biological availability. This problem arises from the fact that many drugs are small organic molecules with low solubility in aqueous fluids. There are several ways to address the solubility problem of poorly soluble drugs.
For example, the drug itself can be modified. The physical form of the drug can be manipulated by various techniques to optimize the rate at which the drug dissolves. Of these techniques, the one most relevant to the present invention is particle size reduction. The rate of dissolution of a solid may often depend upon the surface area that is exposed to the dissolving medium and since the surface area of a given mass of a substance is generally inversely proportional to the substance's particle size, reducing the particle size of a powder or granular substance may increase its dissolution rate.
Where it is effective, particle size reduction increases the dissolution rate of a particulate solid by increasing the surface area that is exposed to the dissolving medium. However, particle size reduction is not always effective at increasing the dissolution rate of a drug from a compressed solid dosage form. Many hydrophobic drugs have a strong tendency to agglomerate during the dosage form manufacturing process into larger particles with an overall decrease in effective surface area. Remington: The Science and Practice of Pharmacy, 20th ed. 656, 657 (A. R. Gennaro Ed., Lippincott Williams & Wilkins: Philadelphia 2000), incorporated by reference herein, contains a more thorough discussion of the concept of “effective surface area” and the effect of particle size on dissolution. A drug that has ostensibly been milled to a fine particle size will sometimes display dissolution characteristics of a larger particle due to agglomeration or similar effect.
There are three well known processes for manufacturing compressed solid dosage forms: the wet granulation method, the double-compression method (also known as dry granulation) and the direct compression method. In each of these methods, there are blending steps which can promote agglomeration of fine particles of the drug into larger, less rapidly dissolving, particles.
In the wet granulation method, pre-weighed drug and one or more other ingredients, like a diluent, are blended. The blend is then mixed with a liquid such as water or ethanol which causes the particles to agglomerate into a damp mass. Sometimes the liquid contains a binder. The damp mass is screened to produce granules which are then dried. The dry granules are screened to produce granules of a predetermined size. Then, the granules are typically blended with a solid lubricant and possibly other ingredients. Lastly, the lubricated granules and any other extra-granular ingredients are compressed into a tablet, which may subsequently be coated.
The double-compression or dry granulation method has fewer steps than wet granulation and does not require contact with a liquid or drying, which makes it well suited for formulating water sensitive and heat sensitive drugs. In the double-compression method, the drug and other ingredients, such as a lubricant, are blended and then compressed in a first compression step. There are two conventional first compression techniques. One is roller compaction where the blend is fed between rollers which press it into sheets and the other is slugging where the blend is compressed into slugs, which are tablet-like forms that are typically larger than tablets intended for human consumption. The resulting sheets or slugs are then comminuted into granules, mixed with a solid lubricant and compressed in a second compression step to produce the final tablet.
The direct compression method is the simplest of the three well known methods for making compressed solid dosage forms. In the direct compression method, the drug and any other ingredients are blended together and directly compressed into the final tablet. The tablet ingredients must have good flow properties and cohesion to be suitable for direct compression tabletting. Microcrystalline cellulose and lactose are two commonly used diluents in direct compression tabletting.
U.S. Pat. No. 6,458,811, incorporated by reference herein in its entirety, describes a pharmaceutical formulation comprising raloxifene in particulate form, said particles having a mean particle size of between about 5 and about 20 microns, at least about 90% of said particles having a size of less than about 35 microns.
In addition to modifying the physical characteristics of the drug, the composition of the dosage form can be adjusted to promote dissolution of the drug. For example, U.S. Pat. No. 5,972,383, incorporated by reference herein in its entirety, describes an orally administrable pharmaceutical formulation comprising raloxifene and a hydrophilic carrier composition. The hydrophilic carrier composition contains a surfactant, a water-soluble diluent, and a hydrophilic binder.
Our experimental work has been guided by a search for ways to modify the composition and method of manufacturing of oral dosage forms to promote dissolution of poorly soluble drugs. Our experimental work also focused on the search for novel manufacturing processes and compositions which enable us to utilize larger drug particles than recommended in the literature, while maintaining a fast dissolution rate and concomitant high bioavailability. The drug raloxifene has been used as a model in much of this work. However, it its more general application, the invention disclosed herein is not to be construed as limited to raloxifene solid dosage forms.
In a well known study published in 1963, Levy et al. reported on the effect of starch on the rate of dissolution of salicylic acid from tablets manufactured by double compression. Levy, G. et al., J. Pharm. Sci. 1963, 52, 1047. It was discovered that increasing the starch content from 5 to 20% increased the rate of dissolution of salicylic acid three fold. This observation was attributed to faster disintegration of tablets with a higher starch content. In 1967, Finholt et al. observed that fine starch particles added to phenobarbital tablets increased the dissolution rate of phenobarbital from the tablets. Reaching a different conclusion from Levy et al., it was proposed that the starch worked by coating the phenobarbital crystals and imparting a hydrophilic property to them, which improved contact between the phenobarbital particles and an aqueous dissolution medium. Finholt, P. Medd Norsk Farm. Selsk 1966, 28, 238.
Starch is a common ingredient of tablets, where it is used for a variety of purposes. It is routinely used, e.g., as a diluent, binder, disintegrant, and glidant. Diluents increase the bulk of a solid pharmaceutical composition and can make a pharmaceutical dosage form containing the composition easier for the patient and caregiver to handle. Binders help bind the active ingredient and other ingredients together, for example, during granulation or compression steps. Disintegrants accelerate break up of the tablet in a patient's stomach, typically by drawing water into the tablet and causing it to swell, thereby breaking the tablet into smaller pieces (resulting in greater surface area). Glidants improve the flowability of powder compositions by coating the surfaces of the particles. According to the Handbook of Pharmaceutical Excipients 4th Ed. 603-604. (Pharmaceutical Press: London 2003), incorporated by reference herein in its entirety, starch is commonly used in an amount of 5-15% when it functions as a binder. (All percentages, unless otherwise specified, are percentage by weight based on the total weight of the compressed solid dosage form.) When functioning as a disintegrant, it is commonly added in an amount of 3-15%. Id. The amount of diluent that is called for in a particular application depends upon many parameters and is highly variable. However, as the Handbook notes, starch does not compress well and tends to increase tablet friability and capping if used in high concentrations. Id. Thus, the use of high concentrations of starch as a diluent is limited by the deterioration in the hardness and friability (resistance to chipping) that occurs as the proportion of starch in the formulation is increased.
It would be highly desirable, therefore, to produce a compressed solid dosage form for oral administration having a high rate of dissolution of a poorly soluble drug without having to reduce the particle size of the drug beyond that size which is predicted by surface area calculation due to agglomeration effects.